Rate constants for the reactions of hydroxyl radicals and chlorine

Ranajit K. Talukdar, Abdelwahid Mellouki, James B. Burkholder, Mary K. Gilles, Georges Le Bras, and A. R. Ravishankara. The Journal of Physical Chemis...
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J. Phys. Chem. 1993,97,4683-4688

Rate Constants for the Reactions of Hydroxyl Radicals and Chlorine Atoms with Halogenated Aldehydes D. J. Scollard, J. J, Treacy, and H. W. Sidebottom' Department of Chemistry, University College Dublin, Dublin, Ireland

C. Balestra-Garcia, G. Laverdet, G. LeBras, H. MacLeod, and S. Thton Laboratoire de Combustion et Systtmes Rbactifs, CNRS, 45071 Orlbans Cedex 2, France Received: October 21, 1992; In Final Form: January 26, I993

Rate constants for the reactions of O H radicals and chlorine atoms with halogenated aldehydes of the type CX3CHO (X = H, C1, or F) have been determined at 298 f 2 K. The O H radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreased overall reaction enthalpy.

Introduction

CH30N0

Halocarbons containing hydrogen atoms may be degraded in the troposphere by reaction with hydroxyl radicals. The resultant oxidation products could be relatively stable in the troposphere and hence provide a flux of halogen-containingcompounds into the stratosphere. All availableevidence suggests that haloethanes with thestructureCX3CH3(X=C1orF)cangiverisetoformation of the corresponding aldehydes, CX3CHO.I Halogenated aldehydes, formed as primary products in the OH-initiated oxidation of halocarbons,may further degradevia reaction with OH radicals or by photolysis. Kinetic data for the reaction of OH radicals with various chlorinated acetaldehydes have been reported from both absolute2J and relative rate studies.4~5 D6M et ale2have also measured the rate constant for the reaction of OH radicals with trifluoroacetaldehyde. The results from these investigations indicate that halogen substitution in CH3CH0 reduces the reactivity with respect to reaction with OH radicals. The aim of the present study was to determine rate data for the reaction of OH radicals with a series of halogenated aldehydes. To provide further information concerning these reactions, rate data for the corresponding reactions with C1 atoms were also obtained.

Experimental Section Relative Rate. Kinetic experiments were carried out at 298 f 2 K and atmosphericpressure, 730-750 Torr, in an approximately 50-L FEP Teflon cylindrical chamber. The chamber was surrounded by 20 fluorescent lamps (10 blacklamps, Philips TL 20/08, and 10 sunlamps, Philips TL 20/09). These lamps provided UV-visible radiation in the region 300-450 nm with an intensitymaximum around 360nm. Thelight intensity wasvaried by switching off various sets of lamps. An electric fan positioned below the reaction chamber helped maintain a uniform reaction temperature during irradiation of the reactants. To prevent prephotolysis of the reactants, the reaction chamber was covered prior to commencing irradiation. Hydroxyl radicals were produced by the photolysis of methyl or ethyl nitrite in air, e.g., 0022-3654f 9312097-4683$04.00/0

+ hv(X > 300 nm)

-

+ 0, HO, + N O

CH,O

-

CH,O

-.

OH

CH30

+ HO,

+ NO,

-

+ NO

(1) (2)

(3)

and chlorine atoms by the photolysis of molecular chlorine. C1,

+ hv(h > 300 nm)

2C1

(4) Measured amounts of the reagents were flushed from Pyrex bulbs into the reaction chamber by a stream of zero-grade nitrogen (Air Products), which was then filled with either zero-grade nitrogen or ultrapure air (Air Products). Quantitative analyses were carried out using gas chromatography (Gow Mac Series 7 50 gas chromatographequippedwith a flame ionizationdetector) and/or FTIR spectroscopy (Mattson Galaxy 5000, 0.5-cm-' maximum resolution). Gastight syringes (Hamilton) or a Valco gas sampling valve were used to remove samples of the reaction mixtures from the Teflon bag for GC analysis, while FTIR spectra were obtained using an evacuable 2-L Wilks cell containing a multipass White mirror arrangement (maximum path length 14 m) and mounted in the cavity of the spectrometer. Laser Photolysis Resononce Fluorescence. The experimental apparatus used in Orldans have been described in detail in previous publication^^^^^^ and will only be briefly outlined. An excimer laser (Lambda Physik LPX 100) with a pulsed output at 248 nm (KrFexciplex) wasused togenerateOHradicals by the photolysis of nitric acid in argon. The pulsed laser beam was directed into a cross-shaped Pyrex reactor (volume 600 om3 approximately), fitted with a Pyrex jacket, through which an appropriate heat exchange fluid could be circulated for temperature variation and control. An OH resonance lamp situated perpendicular to the laser beam was used as a cw source for resonant fluorescence detection of the photolytically generated OH radicals. The lamp and laser beams were collimated and directed toward the reaction zone at the center of the cell. A photomultiplier tube (Hamamatsu R292) mounted above the cell, perpendicularto the intersectionof the lamp and laser beams, 0 1993 American Chemical Society

4684 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993

-

2E4

9)

C C

a

0

E 0

8n

(€4

Y

a

8 \

2p

moo

0 C

.Y

9)

0 C

z

;4000 0 7

ii

0

I

I

I

I

I I

2

48

6

8

I

0

I

I I

10

I I

12

Time /ms Figure 1. OH fluorescencedecay signal ([HNO,] = 6.9 X lOI4 molecule = 2.0 X lOI4 molecule ~ m - k"= ~ , 359 SKI,298 K) for ~ m - [CF3CHO] ~, the reaction of OH radicals with CF3CHO.

-

was used to detect the 308-nm resonance fluorescence arising from the OH A2Z+ X2H 0,O vibrational level transition. Scattered laser light was removed using appropriate filters. The output from the PMT was amplified prior to acquisition by a multichannel analyzer-signal averager (ATNE operating over 256 channels with 100 /.cs per channel) and was linked to a microcomputer for data manipulation and storage. The OH temporal profiles were generated by summing the signals from 512 laser shots with a typical repetition rate of 4 Hz, giving a detection limit for OH radicals of around 1O1Oradicals ~ m - ~ . A typical fluorescence decay signal is shown in Figure 1. The signal decays were found to be exponential over at least 3 halflives. The OH decay rate was determined by fitting an exponential curve to the experimental decay curve [derived from a leastsquares analysis of the data plotted as In (fluorescence intensity) against time]. Prior to commencing each set of experiments, fresh mixtures of nitric acid and reactant were prepared in separate bulbs of accurately known volumes, using argon gas as diluent. The concentrations of nitric acid and reactant were determined manometrically using two cross-calibrated capacitance manometers (MKS Baratron) and were premixed with argon (Alphagaz >99.993%) before entering the cell. The argon flow rate into the cell was controlled using a mass flow controller (Tylan FC260). Cell pressure was monitored using a capacitance manometer (MKS Baratron) and was determined by the flow rate of argon diluent (6-30 L/h) and by adjusting a throttle valve on the pumping side of the cell. Experiments at 298 K were carried out under slow flow conditions at pressures in the region 15-100 Torr with correspondinglinear flow rates of 60-1 5 cm s-I . Laser fluence was varied in the range 150-300 mJ/pulse cm-2 (estimated as 15-30 mJ/pulse cm-2 in the reaction zone), and repetition rate varied in the range 0.25-4 Hz. Materials. Nitric oxide and chlorine were Matheson research grade and were used as received. Organic reagents used as reference compounds in the relativerate studieswere from Aldrich (purity >99%) and were vacuum distilled prior to use. Methyl nitrite was prepared by dropwise addition of 50% H2S04 to methanol saturated with sodium nitrite, while ethyl nitrite was prepared in the same way using ethanol in place of methanol.

Scollard et al. Nitric acid was prepared by distillation from mixtures of NaN03 in excess 98% H2S04 under vacuum. The product was collected at 195 K (ethanol/liquid N2 slush). Acetaldehyde and trichloroacetaldehyde (Fluka >99.5%) were used following vacuum distillation. Chloroacetaldehyde was obtained from a 50%aqueous solution (Aldrich) by fractional distillation and was checked for purity by comparison of the infrared spectrum with literature.* Chlorodifluoroacetaldehyde and trifluoroacetaldehydewere prepared from the corresponding hydrates (Fluorochem), and dichloroacetaldehyde was prepared from the corresponding diethyl acetal (Aldrich) by treatment with concentrated sulfuric acid according to the method of Pierce and Kaneqg Literature infrared spectra of chlorodifluoroacetaldehydeloand trifluoroacetaldehydel I were compared with those of the prepared samples. Dichlorofluoroacetaldehyde, chlorofluoroacetaldehyde,and difluoroacetaldehyde were prepared by treatment of the hemiacetal/hydrate mixtures. These compounds were obtained by reduction of the esters CC12FCOOCH3, CHClFCOOC2Hs (Fluorochem), and CHF2COOC~HS(Fluorochem) by treatment with LiAIH4 at low temperature, according to the method of Yamada et al.I2 The identity of dichlorofluoroacetaldehydewas confirmed by comparison of the infrared spectrum with that of Yamada et al.I2The ester, CC12FCOOCH3,was prepared from CC13COOCH3(Aldrich) according to the method of Gryszkiewicz-Trochimowski et al.,I3 and the IH and 13CNMR spectra of the prepared ester were compared with those of Yamada et al.Iz All prepared samples were purified by repeated trap-to-trap distillation under vacuum and were analyzed by gas chromatography and infrared spectroscopy. Pure samples of these halogenated aldehydes rapidly undergo polymerization at room temperature and were therefore stored at 253 K in the dark. RWdQ

Relative Rate. Photolysis of C H 3 0 N 0 or C2H50NO/NO/ halogenated aldehyde/reference compound mixtures was carried out in air at 298 f 2 K and 1-atm total pressure: [CH30NO]o or [C~HSONOIO = 10-30ppm; [NO10 = 0-50 ppm; [CX3CHOIo = 5-30 ppm; [Reflo= 5-30 ppm (1 ppm = 2.46 X 1013 molecule ~ m at- 298 ~ K and 740-Torr total pressure). Relative rate constants for reaction of the aldehydes with OH radicals were determined by comparison of the decay of the reactant aldehyde, CX3CH0, with that of the reference compound.

-

+ CX3CH0 OH + reference

OH

products

(5)

products

(6)

Assuming that reaction with OH is theonly significant loss process for both reactant and reference compounds, it can be shown that

In( [CX,CHO],/(CX,CHO],) =

kdk, ln([Refl,/[Refl,) (1) where the subscripts zero and t indicate concentration at the beginning of the experiment and at time t , respectively. The rate constant ratios, k5/k6, were determined from plots of eq I, and the rateconstants ks werederived from evaluated literaturevalues of k6.14 The reference compounds used were C2H4 for CH3CHO, k6 = 8.52 X 10-12cm3molecule-I s-I; CH3COCH2CH3for CH2ClCH0and CHC12CH0, k6 = 1.15 X 10-12cm3molecule-' s-l; C6HsCH3 for CCl3CH0, CC12FCH0, CHCIFCHO, and CHF2CH0, k6 = 5.96 X 10-l2cm3molecule-I SKI;CH3CH20H for CClF2CH0,k6 = 3.27 X 10-12cm3 molecule-' s-1; and CH3COCH3 for CFsCHO, k6 = 2.26 X lCi3cm3 molecule-] s-I. Mixtures of aldehydes and reference compounds with methyl or ethyl nitrite were stable in the dark for several hours in the Teflon reaction chamber. However, photolysis of CHC12CH0, CC13CH0, CC12FCHO. CClF2CH0, and CHClFCHO in air

Reactions of OH Radicals and C1 with Halogenated Aldehydes The Journal ojPhysical Chemistry, Vol. 97, No. 18, 1993 4685 0.4

TABLE I: Rate Constants for the Reaction of OH Radicals with Halogenated Aldehydes at 298 i 2 K

0 - CHC12CH0 0- CCI CHO 3

-

aldehyde

10W-b

CH3CHO

15.0 f 3.8 15.3 i 1.6 16.2 f 1.8 16.0 f 1.6 12.8 f 4.3 12.2 f 2.7 14.2 f 1.0 16 f 3d 16.9 f 3.5 17f3 16.2 f 1.0

DF-MS DF-MS RR FP-RF RR FP-RF DF-RF DF-RF LP-RF RR

Morris et al. (1971)'* Morris and Niki (1971)19 Niki et al. (1978)20 Atkinson and Pitts (1978)21 Kerr and Sheppard (1981)22 Semmes et al. ( 1985)23 Michael et al. (1984)z4 Atkinson (1989)14 D6b6 et al. (1989)2 Balestra-Garcia et al. (1992)3 this work

3.2 3.0 f 0.6 3.1 f 0.2

RR LP-RF RR

Starcke et al. (1990)4 Balestra-Garcia et al. (1992)) this work

CHC12CHO >2.8 2.4 f 0.5 2.3 f 0.1

RR LP-RF RR

Starcke et al. ( 1990)4 Balestra-Garcia et al. (1992)) this work

CCllCHO

1.6 f 0.3 >1.2 1.8 f 0.3 0.86 0.17 1.6 f 0.2

DF-RF RR RR LP-RF RR

D6bC et al. (1989)2 Starcke et al. (1990)4 Nelson et al. (1990y Balestra-Garcia et al. (1992)3 this work

a- C C I F ~ C H O

0.3

0 .u

(1

x E

Y

\

0

0.2

0

r

x ::

CHlClCHO

Y

5

0.1

*

0.0

0.3

0.0

1.o

0.8

0.5

technique'

reference

( [Ref lo / [ Ref It I Figure 2. Concentration-time data for the reaction of O H radicals with

CHClFCHO

2.2 f 2.2 2.0 f 0.3

LP-RF RR

this work this work

various halogenated aldehydes.

CCl2FCHO

1.3 f 0.1 1.1 f 0 . 2

LP-RF RR

this work this work

resulted in a fairly rapid decay of the aldehydes. In the absence of 02,the decay was reduced significantly. Similarly, addition of NO or C& was shown to decrease the rate of aldehyde disappearance to negligible amounts over the time scale of the OH radical kinetic experiments. Theseobservationsareconsistent with the suggestion that photolysis of these aldehydes in oxygen leads to a chain reaction involving chlorine atoms,5 e.g.

CClF2CHO

0.95 f 0.05 LP-RF 0.70* 0.05 RR

this work this work

CHF2CHO

1.7 f 0.2 1.4f 0.3

this work this work

CF3CHO

1.1 f 0.7 DF-RF 0.65 f 0.05 LP-RF 0.55 f 0.12 RR

+ hu CClF, + 0, + M

CClF,CHO

2CCIF,O, CCIF,O and

-

+ 0,

COF,

+ -

CCIF,C(O)O

+ 0,

(8)

(10)

+ HCl

CClF,C(O)O,

2CClF,C(O)O CCIF,

(7)

(9)

+ C1

CClF,CO

M

2CC1F2C(O)O,

+ CHO CCIF,O, + M

CCIF,

2CC1F20

-

C1+ CCIF,CHO CClF,CO

-

+M

+ 0,

+ CO,

(1 1) (12)

(13) (14)

Reaction of OH radicals with aldehydes is expected to involve H atom abstraction from the CHO group, e.g. OH

+ CClF,CHO

-

CClF,CO

+ H,O

(5) Hence, reaction with OH will also lead to enhanced loss of the aldehydes via a C1 atom chain reaction. Addition of NO or C2H6 efficiently removes the C1 atoms by reaction to give NOCl and HCI, respectively. Thechain reaction is then unimportant. Thus, reaction with OH will be the dominant loss process for the aldehydes under these conditions. Further, since the CH2ClO radical does not eliminate C1 but reacts with 0 2 to give CHC10,l

LP-RF RR

D6bCet al. (1989)2 this work this work

In units of cm) molecule-' SKI. Errors taken from the literature are often twice the standard deviation and represent precision only. c DFMS, discharge flow mass spectrometry: RR, relative rate; FP-RF, flash photolysis resonance fluorescence; DF-RF, discharge flow resonance fluorescence; LP-RF, laser photolysis resonance fluorescence. d Evaluation.

reaction of OH with CH2ClCHO does not give rise to a C1 atom chain process. Concentration-time data for a number of the aldehydes are shown plotted in the form of eq I in Figure 2. The rate constant ratios, kS/ k6, were independent of reaction time, relative reactant concentrations, and light intensity in agreement with the proposed mechanism. At least five individual runs were carried out with each substrate, and to test the internal consistency of the rate constant ratios, each compound was run against another member of the series. In all cases the relative rate values were in excellent agreement with the results using the reference compounds. The derived rate constants for each aldehyde are given in Table I. The errors quoted are twice the standard deviation, arising from a linear least-squares analysis of the data, and do not include an estimateof theerror in the reference rateconstant. Theevaluated errors in k6I4add about a further 25% to the uncertainty of the rate constants reported in these relative rate studies. In a similar series of experiments molecular chlorine was photolyzed in the presence of halogenated aldehyde/reference compound/N2 mixtures at 298 K with [CX~CHOIO = 5-30 ppm, [Reflo = 5-30 ppm, [C12]0 = 5-20 ppm

-

C1+ CX3CH0

products

(11)

C1+ reference

products

(15)

4686 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993

Scollard et al. 1250

0.3

lo00

0

0.2

8n

750

E

c

-

b

Y

\

0

\

aE 0

-5 u

L 500

- CClF2CH0

0 CHCIFCHO A CHFZCHO A CClzFCHO

0.1

V

0-CF3CH0 I I

0

0.0

0.0

0.2

0.8

0.4

1.0

0.8

I I

I I

1.2

It 1 Figure 3. Concentration-time data for the reaction of chlorine atoms with various halogenated aldehydes. In

I [ Ref lo /

Ref

TABLE Ik Rate Constants for the Reaction of Chlorine Atoms with Halogenated Aldehydes at 298 f 2 K aldehyde CH3CHO

CHzClCHO CHC12CHO CC13CHO CHCIFCHO

10'2ka-b 16 f 6 85 f 8 59f 10 76 f lod 66f 14 79 i 6

technique'

reference

FP-RF RR

Niki et al. (1985)25 Wallington et al. (1988)26 Bartels et al. (1989)27 Atkinson et al. (1989)28 Payne et al. (1990)29 this work

15f3

RR RR

Starcke et al. (1990)4 this work

12.7 13f2

RR RR

Starcke et al. (1990)4 this work

RR RR

Starcke et al. (1990)4 this work

RR

this work

10.8

9.0 7.1 f 0.5 10.0f 2

RR RR

DF-MS

CC12FCHO

5.7 f 1.2

RR

this work

CClFzCHO

4.5 f 0.3

RR

this work

CHFzCHO

5.6 f 1

RR

this work

CF3CHO

2.1 f 0.1

RR

this work

In units of cm3 molecule-' SKI. Errors taken from the literature are often twice the standard deviation and represent precision only. Key as per Table I. Evaluation.

and ln([CX,CHO]o/[CX,CHO],) =

k , , / k , s ln([Reflo/[ReflJ (11) Typical concentration-time data from the C1 atom relative rate studies are plotted in the form of eq I1 in Figure 3. The rate constants given in Table I1 were obtained from the slopes of these plots taking the following values of the rate constants for reaction of C1 atoms with the reference compounds: kls = 1.51 X 1O-"J cm3 molecule-' s-l for (CH3)zO (reference used for CH3CHO);'S kls = 5.59 X 1O-" cm3molecule-' s-I for C6HsCH316(reference used for CH2ClCH0, CHC12CH0, CC13CH0, CHCIFCHO,

cm3molecule-' s-I for CH3and CCLFCHO); k15 = 2.37 X COCH317(referenceused for CHF2CH0, CClF2CH0, and CF3CHO). Laser Photolysis Resonance Fluorescence. The decay of OH in the absence of added reactant is due to the following reactions

OH

-

OH

+ HNO,

-

H,O

+ NO,

loss by diffusion out of the viewing volume

(16)

(1 7)

Since [HN0310 >> [OHIO,first-order kinetics apply, and the temporal profile of OH is described by the expression ln([OH],/[OH],) = (k,,[HN03]o

+ kl,)t = kft (111)

Upon addition of an aldehyde, which reacts with OH under pseudo-first-order conditions ( [CX~CHOIO > 100 [OHIO) OH

+ CX,CHO

-

products

(5)

an additional term must be added to eq I11 ln([OH],/[OH],) = (k,[CX,CHO],

+ k')t = k"t

(IV)

Thus, the bimolecular rate constant, ks, is obtained from the slope of k"against [CX3CHO], which has an intercept k'-the first-order rate constant for loss of OH in the absence of added aldehyde. Initial concentrations of HNO3 were chosen such that k'was approximately 200 s-I and were varied in the range 4-8 X lOI4 molecules cm-,; k17 was typically equal to 30 s-I at 27 Torr. Representativedata plots for the aldehydesstudied, plotted in the form of eq IV, are shown in Figure 4, and the statistically weighted values of the rate constants derived from at least three individual determinations are presented in Table I. The initial concentration of aldehyde was varied over at least a 5-fold concentration range. The concentration ranges used are as follows: 0.6-4.0 X 1014molecule cm-l for CHF2CHO 0.6-3.0 X 1014molecule cm-, for CHCIFCHO; 1.4-8.2 X lof4molecule ~ m for - CC12FCHO; ~ 1.20-9.7 X lOI4molecule ~ m for - CClF2~ CHO; and 1.5-10.0 X 1014 molecule cm4 for CF3CHO.

Reactions of OH Radicals and C1 with Halogenated Aldehydes The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4687

Discussion The OH radical and C1 atom rate constant data for reaction with halogenated aldehydes obtained at 298 f 2 K in this work are compared with previous determinations in Tables I and 11, respectively. The rate constants for reaction of OH with CH3CHO, CH2ClCH0, and CHC12CHO from the relative measurements at l-atm total pressure are in good agreement with those previously obtained in one of our laboratories using LP-RF at approximately 30-Torr total pressure3 and with the relative rate determinations of Starcke et al.4 The two rate constants determined in our laboratories for CC13CH0, using both CH3COOC2H5 and C&CH3 as reference compounds, are within experimental error of those reported by Starcke et al.4 using n-butane as the reference in a relative rate measurement. They are also in good agreement with the value obtained from the discharge flow resonance fluorescence method by D6b€ et a1.2 However, a recent measurement of this rate constant by BalestraGarcia et al.,3 using the pulsed laser photolysis resonance fluorescence technique, is about 50% lower than these values. The relative rate and LP-RF determinations of the OH rate constants for the fluorine-containing halogenated aldehydes in this series are also in good accord, and the data for CF3CHO are within the large experimental error quoted by D6bC et a1.2for this reaction. Thus, with the exception of the data for CC13CH0, the rate constants from the present relative rate study are in satisfactory agreement with those from the LP-RF determinations. The reason for the large discrepancy shown inthe rate data for reaction of OH with CCl3CHO obtained by Balestra-Garcia et al.3 and other measurementsonthis s y ~ t e m is ~ unclear. . ~ - ~ Further work on the OH CC13CH0 reaction is required to fully understand the discrepancies in the measured rate data. Rate data for the reactions of C1 atoms with halogenated aldehydes are given in Table 11. The data for reaction with CH3CHO obtained in this work are in good agreement with both previous r e l a t i ~ eand ~ ~ abs01ute~~J~ .~~ rate studies. Broad agreement with the data of Starcke et al.4 for reaction with the chlorinated aldehydes is also apparent. Examination of the data in Table I for the reaction of OH radicals with the halogenated aldehydes shows that substitution of the methyl hydrogens in acetaldehyde with halogen atoms decreases the rate constant for reaction. The OH radical rate constant for reaction with CC13CH0 determined in this work, and in previous st~dies,2.~,5 is approximately 10 times lower than the rate constant for reaction with CH3CH0, while the rate constant for reaction with CF3CHO is lower still by almost a factor of 3. In an analogous fashion to the OH rate data, the C1 atom rate constant for reaction with CC13CHO is approximately an order of magnitude lower (Starcke et al.4and this work) than with CH3CH0. In addition, the chlorine atom rate constant for reaction with CF3CHO determined in this work is approximately 30 times lower than with CH3CHO. The major reaction pathway for reaction of OH radicals14and C1 atomsZ9with acetaldehyde is hydrogen atom abstraction from the aldehyde group. Presumably the same mechanism is operative for the halogenated aldehydes. Hydroxyl radicals and C1 atoms are both electrophilic species, and correlations between their respective rate constantsfor reactions having the same mechanistic features are to be expected. Figure 5 shows a free energy plot for the OH and C1 reactions with the halogenated aldehydes, using the rate constant values determined in our laboratories. As expected, the plot is essentially linear and appears to provide support for the higher values of the rate constants for reaction of OH with CCl3CHO. The deactivation of the aldehydic hydrogen with respect to abstraction following chlorine or fluorine substitution in the CH3 group can be rationalized in terms of changes in the overall enthalpy of reaction and/or destabilizing polar effects in the transition state.

+

RR OH data LP-RF OH data

-27.5

1 1

-26.8

-25.8

1 I

-24.8

1

I

-23.8

In kCI Figure 5. Linear free energy plot for the halogenated aldehydes.

The aldehydic C-H bond strength in CH3CHO is 355 kJ ~ O ~ - Iand , ~ Othus reactions with OH radicals and C1 atoms are both strongly exothermic, D{H-OH] = 491 kJ mol-I 3O and D{H-CI) = 425 kJ mol-1.30 Bond dissociation energies have not been determined for the halogenated aldehydes with the exception of CFsCHO, D(CSC(O)-H) = 381 kJ Available thermochemicaldata30suggest that halogenation on the 0carbon does not generally change C-H bond energies. Therefore, it is reasonable to assume that, with the exception of CF3CH0, halogenation will not sufficiently strengthen the CX3C(O)-H bond to account for the observed reactivity trends. Further, the calculated overall exothermicities for reaction of OH radicals with CH3CHO and CF3CHO are 136 and 110 kJ mol-], respectively; thus, a small change in the overall reaction exothermicity would be unlikely to account for a decrease of almost a factor of 30 in the OH rate constant for reaction with CF3CHO. It is suggested that the relatively largechanges in the reactivity are mainly due to inductive effects in the transition states. The transition states of highly exothermic reactions are normally assumed to be similar in structure and energy to the reactants.” Therefore, the considerable charge separation in halogenated aldehydes32will be reflected in the loose transition states formed in reactions with OH radicals and C1 atoms. The transition state involves a hydrogen positioned between the attacking electrophilic OH radical and the CX3C(0) radical. Chlorine or fluorine substitution of the CH3 group will reduce the electron density on the H atom and thus destabilize the transition state compared to that formed with CH3CHO. Furthermore, these transition states will be increasingly destabilized as the degree of the charge separation increases, Le., as the electron withdrawing ability of the halomethyl group increases. Thus, as expected, the reduction in reactivity upon substitution by the highly electronegative fluorine atom is greater than that observed upon chlorine atom substitution. Further support for the importance of polar effects in the reaction of radical species with halogenated aldehydes comes from the reactions of CH3 and CF3 radicals with CH3CHO and CF3CH0. For the electrophilic CF3 radical, the rate constant at room temperature for reaction with CH3CHO is about 2 orders of magnitude higher than that for reaction with CF3CHO, whereas the rateconstants for reaction with the nucleophilic CH3 radical are almost equal.33 The lifetimes of the halogenated aldehydes studied in this work with respect to removal by OH radicals in the troposphere are

Scollard et al.

4688 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993

Although halogenated formaldehydesare probably the major products from both the photolysis and OH radical initiated oxidation of halogenated aldehydes, it is possible that stable compounds of structure CX~C(0)02N02may result following hydrogen atom abstraction from halogenated aldehydes by OH radicals. Hence, the relative importance of CX3CHO photolysis and reaction with OH radicals must be determined under atmospheric conditions to evaluate the role of halogenated acyl peroxynitrates as reservoir species for chlorine and fluorine atoms.

TABLE IIk Atmospheric Lifetimes for Halogenated Aldehyde8 with Respect to Reaction with OH Radicals aldehyde 10'2ko~(298K)' lifetime: days CHiCHO CH2CICHO CH C 12C H 0 CCliCHO CHClFCHO CClzFCHO CCIF2CHO CHFlCHO CFiCHO

16.2 3.1 2.3 1.6 2.1 1.2 0.7 1.4 0.55

0.83 4.3 5.8 8.3 6.3 11 19 9.5 24

Data from this workin cm3molecule-I s-l. Lifetime = l / k o ~ [ O H ] with a 24-h averageglobal tropospheric [OH] = 8.7 X lo5radical~m-'.)~

given in Table 111. The data show that all of the aldehydes have relatively short lifetimes. Absorption cross-section measurements for acetaldehyde and chl~roacetaldehydes~ indicate that the photolysis rates for these compounds will be comparable to OH radical removal rates. Thus, the halogenated aldehydes will not provide an important flux of C1 atoms into the stratosphere. It is of some interest to speculate on the fate of the products arising from the photolysis and the OH radical initiated oxidation of halogenated aldehydes under atmospheric conditions. Reaction with OH radicals gives rise to CX3CO radicals, while it is expected that photolysis will yield the CX3 radicalas

OH

- + + - + + -cx, + + + + + +

+ CX,CHO

hv

CX,CHO CX,C(O)O,

CX,CO CX,

CHO

NO

CX,C(O)O

cx,c(o)o

CO,

CX,

CX,O,

0,

M

NO

CX,O,

CX,O

H,O

(7)

+ NO2

(18) (14)

M

(8)

NO,

(19)

The available evidence suggeststhat halogenated methoxy radicals yield the corresponding halogenated formaldehydes, with the exception of CFsO radicals, for which the atmospheric fate is unclear.' The fully halogenated formaldehydesare photolytically stable in the troposphere, and reaction with OH radicals is of no importance. It is believed that their major fate in the troposphere is aqueous phase hydrolysis,' although data on this sink are somewhat limited. It is possible that these species could contribute to stratosphericchlorine atom concentration^.^^ Under conditions of high NO2 to NO concentrations, the peroxy radicals, CX3C(0)02 and CX3O2, may form the corresponding peroxynitrates. CX,C(0)02

+ NO, + M

-

CX,C(0)0,N02

CX,CO,

+ NO, + M

-

CX,02N0,

+M

+M

(20) (21)

Kirchner et have recently investigated the thermal stability of severalhalogenated alkyl and acyl peroxynitrates. Their results suggest that the PAN-like structures CX,C(0)02N02 have extremely long thermal lifetimes under conditions typical of the upper troposphere. Hence, the lifetimes of these species will be limited by their photolysis rates, which have not been determined to date. In contrast, the halogenated alkyl peroxynitrates were shown to have lifetimes with respect to thermal decomposition of only a few days in the upper troposphere and hence will not play an important role as reservoirs for halogen atoms.

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